Solid acid catalysts have been used commercially for oligomerization of olefinic feedstock. In an oligomerization process monomers are converted to a finite degree of polymerization. In processes using olefinic feedstock, light olefins (C3= to C5=) are converted typically into branched olefins in the C6-C15 range using solid phosphoric acid catalyst (sPa). The sPa process was developed by UOP in the 1930's. This process has a number of drawbacks: (1) low catalyst life due to pellet disintegration causing reactor pressure drop; (2) environmental waste handling problems; and (3) operational and quality constraints limit flexible feedstock. Previously it has been found that acidic zeolites with 10-ring pores, such as ZSM-22, ZSM-23, and ZSM-57, are good alternative catalysts for olefin oligomerization, wherein branched higher olefins are produced from light olefins. These branched higher olefins are further derivatized to branched (OXO) alcohols which in turn are esterified to produce esters that are used as plasticizers. Additionally, these branched higher olefins are hydrogenated to produce desired hydrocarbon solvents. Further, these lightly branched higher olefins are useful in alkylation of benzene or phenol to produce sulfonate detergent precursors. Zeolite technology offers several advantages compared with the older sPa technology including ease of handling, higher catalytic activity, improved product selectivity, and facile catalyst regeneration capability.
U.S. Pat. No. 5,026,933 (Blain et al.) discloses the use of 10-member ring zeolites for higher olefin production. That is, heavy distillate and lubricant range hydrocarbons can be synthesized over ZSM-5 type catalysts at elevated temperature and pressure to provide a product having substantially linear molecular conformations due to the ellipsoidal shape of these catalysts. Conversion of olefins to gasoline and/or distillate products is disclosed in U.S. Pat. Nos. 3,960,978 and 4,021,502 (Givens, Plank and Rosinski) wherein gaseous olefins in the range of ethylene to pentene, either alone or in admixture with paraffins are converted into an olefinic gasoline blending stock by contacting the olefins with a catalyst bed made up of a ZSM-5 type zeolite. Such a technique has been developed by Garwood, et al, as disclosed in European Patent Application No. 83301391.5, published 29 Sep. 1983. In U.S. Pat. Nos. 4,150,062; 4,211,640; 4,227,992; and 4,547,613 Garwood, et al. disclose operating conditions for a process for selective conversion of C3+ olefins to mainly aliphatic hydrocarbons. In the process for catalytic conversion of olefins to heavier hydrocarbons by catalytic oligomerization using a medium pore shape selective acid crystalline zeolite, process conditions can be varied to favor the formation of hydrocarbons of varying molecular weight. At moderate temperature and relatively high pressure, the conversion conditions favor C10+ aliphatic product. Lower olefinic feedstocks containing C2-C8 alkenes may be converted; however, the distillate mode conditions do not convert a major fraction of ethylene. A typical reactive feedstock consists essentially of C3-C6 mono-olefins, with varying amounts of non-reactive paraffins and the like being acceptable components. One conventional process for producing substantially linear hydrocarbons by oligomerizing a lower olefin at elevated temperature and pressure comprises contacting the lower olefin under polymerization conditions with siliceous acidic ZSM-23 zeolite having Bronsted acid activity; wherein the zeolite has acidic pore activity and wherein the zeolite surface is rendered substantially inactive for acidic reactions, the zeolite surface being neutralized by a bulky trialkyl pyridine compound having an effective cross-section larger than the zeolite pore.
Although higher olefins produced from zeolite-based catalysts have lower branching than those made with sPa, it is highly desirable to further reduce branching of higher olefin product streams. It is known that collidine (2,4,6-trimethyl pyridine) is an effective agent to deactivate surface acid sites of 10-ring zeolites, thus improving catalyst selectivity toward production of less-branched higher olefin products from olefin containing feedstocks. However, a drawback of collidine is its tendency to desorb from the surface under reaction olefin oligomerization conditions. Desorption is especially troublesome if the oligomerization reaction temperature is higher than 240° C. For olefin oligomerization process, typical commercial end of cycle temperature is about 250° C.
Collidine co-boils with 1-decene and desorbed collidine could contaminate higher olefin products and derivatives, such as branched alcohols produced in a down-stream OXO process. In order to maintain a constant level of collidine on zeolite, a continuous co-feeding of collidine with feed olefin is required. Another drawback of organic-based surface treatment is collindines inability to survive air-regeneration of the spent catalyst. That is, air regeneration burns off the organics, such as collidine. The inorganic species (such as zeolites, yttria and La-oxide) remain intact.
Accordingly, the composition of surface modified 10-ring zeolite catalysts requires the control of the surface acidity of the catalyst to enable a product higher olefin containing stream with low branching levels per molecule. For example, one such technique is to treat the 10-ring zeolite catalyst with an organic base such as collidine. However, because the collidine modified zeolite catalyst is not thermally stable at end of run temperatures, there is leaching of collidine and potentially contamination of the higher olefin containing product stream. Moreover, the high temperature air regeneration of collidine modified catalyst leads to decomposition of the collidine. Therefore, collidine treatment has to be repeated after each air regeneration before the catalyst can be used for higher olefin production.
There is a continuing need for improvement in the catalyst for olefin oligomerization reactions of the type described above. In particular there is a need for effective surface modified zeolite catalysts such that they are stable to end of run olefin oligomerization temperature, do not leach an organic base into the higher olefin product stream and are air regenerable.
The present disclosure provides a novel alternate catalyst system for olefin oligomerization to lightly branched higher olefins, stable to end of oligomerization reaction temperatures, stable to air regeneration, and does not leach organic base to contaminate the higher olefin product stream.